1. Field of the Invention
The present invention relates to a sound dampening device and method, and more particularly, to an active noise cancellation device and method for reducing fan blade noise over a wide spatial area.
2. Discussion of the Prior Art
Noise, in general, can be defined as any sound that is undersired or interferes with one's hearing of something. It is well known that continued exposure to certain levels of noise can cause individuals varying levels of distress, ranging from minor discomfort to serious pain and permanent physical injury in the form of hearing loss. Prolonged exposure to noise levels below approximately seventy decibels is sustainable to the majority of individuals. Prolonged exposure to noise levels which are in the range of approximately seventy to ninety decibels typically causes individuals to experience irritation and stress. Sustained exposure to noise levels in the range of approximately ninety to one hundred twenty decibels can cause permanent hearing loss, and exposure to noise levels in excess of one hundred twenty decibels can reach the threshold of pain for the majority of individuals. Accordingly, when possible it is generally desirable to reduce noise levels as much as possible in a particular environment.
While exposure of limited duration to noise is part of the overall problem of noise pollution, environments where individuals are exposed to noise for extended periods, for example the work place, pose a more serious concern. The typical work place can be a factory with heavy machinery operation or a modern office with computers, word processors, and typewriters. Factories utilizing heavy machinery generate much higher levels of noise than modern offices; accordingly, much of the focus of noise reduction has centered on quieting factory type environments. However, although the noise levels in modern offices are much lower than in factories utilizing heavy machinery, the noise generated by office machines and computer work stations do present a definite noise problem.
One of the significant contributors to the noise problem in offices is fan noise. The majority of electrically powered machines utilized in offices require fans to supply air for cooling purposes. Computers and word processors all require fans to deliver cooling air to the electronics to prevent damage thereto. Other examples of devices utilizing fans are overhead projectors and photostatic copying machines. Therefore, in order to eliminate a significant contributor to the noise problem, in and out of the work place, some form of noise reduction is required to reduce fan noise.
The science of noise reduction can be divided into two broad categories, passive noise reduction and active noise reduction. Passive noise reduction involves the blocking of the compression waves generated by the noise source with a sound absorbing device. This technique is labeled as passive noise reduction because it does not require an external energy source to accomplish its task. Passive noise reduction techniques tend to be more effective for higher frequency noise than for lower frequency noise. There are many well known passive noise reduction devices, for example, automobile mufflers, acoustical wall and ceiling tiles, and a wide assortment of enclosure devices for noisy machines. Active noise reduction techniques, in contrast, refers to any electro-acoustical method in which an undesired sound wave is cancelled by a second sound wave that has the same spatial geometry amplitude and frequency, but is one hundred eighty degrees out of phase. Accordingly, an undesired sound can be cancelled by generating a second sound of the same amplitude and frequency, and adjusting its phase so that the peaks of one sound wave coincide with the valleys of the second wave thereby resulting in destructive interference. Active noise reduction techniques tend to be more effective in attenuating lower frequency noise and vibration. Accordingly, active and passive noise reduction techniques have been most effectively utilized in a complementary fashion to attenuate a variety of wideband noises.
There exists elaborate devices for active noise reduction as evidenced by an examination of the patent art. U.S. Pat. No. 5,224,168 to Martinez et al. discloses a method and apparatus for the active reduction of noise and other compression waves. The patented invention utilizes multi-channel noise reduction techniques in conjunction with signal processing techniques to achieve the desired results. The apparatus is a complex system comprising microphones, a multi-channel signal processor, speakers, and various filtering devices. Essentially, the apparatus operates by generating a number of compression signals from compression waves detected by the microphones at a number of locations within a particular medium. The compression signals are processed by the multi-channel signal processor in order to produce complementary compression waves which are then directed towards the noise through the speakers. Neutralization filters are utilized to compensate for the feedback which occurs when speakers and microphones are used in close proximity.
U.S. Pat. No. 5,140,640 to Graupe et al. discloses an apparatus for cancelling or substantially reducing the noise from a source. The patented invention is a self-adaptive noise cancellation system that is utilized in a noisy environment in proximity to the noise source to produce anti-noise signals that are directed to the noise and which counter the noise source. The system comprises a first microphone which captures the noise signal and directs the noise signal to a stochastic identifier circuit. The stochastic identifier circuit generates a set of stochastic parameters that characterize the noise signals from the source. The stochastic identifier circuit periodically updates these parameters to make the system adaptive and self-adjusting. A noise cancellation circuit generates an anti-noise signal or cancellation signal which is directed to a loudspeaker positioned in proximity to the noise source. The cancellation signal combines with the noise from the source to substantially reduce the noise level. The noise cancellation circuit receives a set of signals from the stochastic identifier circuit as one input and a set of signals from an amplifier circuit configured as a summing circuit as a second input and generates the cancellation signal therefrom. The amplifier circuit combines the signals captured by the first microphone and the signals captured by a second microphone positioned in proximity to the noise source. The second microphone is utilized to capture the anti-noise signal.
Each of the above two described inventions disclose devices and techniques for the effective reduction of noise created by a variety of sources, including the noise generated by the movement of fan blades through the air. However, the above-described inventions, like other commercially available active noise reduction devices, utilize digital signal processing techniques and complicated systems to reduce noise. Devices such as these which require relatively complicated electronics and digital signal processing are expensive and difficult to implement and this would not typically be utilized on smaller electrically powered, fan cooled machines. In addition, in order to achieve noise cancellation over a wide spatial area, cancellation should occur at the source.